Mechanically flexible and durable substrates and method of making

ABSTRACT

A flexible substrate are disclosed comprising an amorphous inorganic composition, wherein the substrate has a thickness of less than about 250 μm and has at least one of: a) a brittleness ratio less than about 9.5 (μm) −1/2 , or b) a fracture toughness of at least about 0.75 MPa·(m) 1/2 . Electronic devices comprising such flexible devices are also disclosed. Also disclosed is a method for making a flexible substrate comprising selecting an amorphous inorganic material capable of forming a substrate having a thickness of less than about 250 μm and having at least one of: a) a brittleness ratio of less than about 9.5 (μm) −1/2 , or b) a fracture toughness of at least about 0.75 MPa·(m) 1/2 ; and then forming a substrate from the selected inorganic material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and claims the benefit ofpriority under 35 U.S.C. § 120 of U.S. application Ser. No. 15/840,888,filed on Dec. 13, 2017, which in turn, is a divisional application andclaims the benefit of priority under 35 U.S.C. § 120 of U.S. applicationSer. No. 15/232,401, filed on Aug. 9, 2016, which in turn, is adivisional and claims the benefit of priority under 35 U.S.C. § 120 ofU.S. Pat. No. 9,434,642, granted on Sep. 6, 2016, the contents of eachof which are relied upon and incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to flexible substrates and methods used inthe manufacture of flexible substrates.

Technical Background

Flexible substrates can be used in a variety of applications includingelectronic devices, such as, for example, light emitting displays. Insuch applications, flexible substrates can be subjected to tensile,compressive, and shear stresses during manufacture, handling, andoperation that can result in device failure or a reduction in thelifetime of a device. The mechanical requirements and thus, theselection and/or manufacture of appropriate substrate materials, canvary depending on the intended application. Several factors typicallyconsidered in evaluating substrate materials include: mechanicaldurability, process compatibility, weight, bend radius, thermalcapability, surface roughness, transparency, electrical properties, andcost.

Various materials have been used in the manufacture of flexiblesubstrates and devices. Metal substrates, such as, for example,stainless steels, typically exhibit properties, such as, for example,surface roughness, non-transparency, and conductivity, that areincompatible with at least some light emitting display devices.Similarly, thermoplastic substrates, such as, for example, polyethylenenaphthalate, polyethersulfone, polycarbonate, and polyimide, can exhibitoxygen and water barrier properties, coefficients of thermal expansion,thermo-mechanical stability, thermal limitations, and chemicaldurability properties that are incompatible with at least some lightemitting display devices. While inorganic film coatings can be employedto alter the barrier properties of thermoplastic substrates, these thinfilms are typically brittle and are prone to cracking, thus resulting inpermeability and/or device failure.

Substrates comprised of glass materials have traditionally been selectedbased on available materials and extrinsic properties such as, forexample, thickness. The glass materials typically selected can exhibitpoor mechanical stability as a result of brittleness and/or poormechanical durability that are not sufficient to withstand the devicemanufacturing process and/or use in the final application.

The size and durability requirements for electronic devices arecontinuously increasing. Thus, there is a need to address dimensionalstability, coefficients of thermal expansion, toughness, transparency,thermal capability, barrier and hermetic properties, and otherproperties of flexible substrates related to use in electronic devices.These needs and other needs are satisfied by the composition and methodsof the present invention.

SUMMARY OF THE INVENTION

The present invention relates to flexible substrates and specifically tomechanically durable flexible substrates comprising an amorphousinorganic composition that can be used in, for example, electronicdevices, such as light emitting displays. The present inventionaddresses at least a portion of the problems described above through theuse of novel compositions, selection criteria, and/or methods ofmanufacture.

In a first aspect, the present invention provides a substrate comprisingan amorphous inorganic composition, wherein the substrate has athickness of less than about 250 μm and at least one of: a) abrittleness ratio of less than about 9.5 (μm)^(−1/2), or a fracturetoughness of at least about 0.75 MPa·(m)^(1/2).

In a second aspect, the present invention provides an electronic devicecomprising a flexible substrate comprising an amorphous inorganiccomposition, wherein the substrate has a thickness of less than about250 μm and at least one of: a) a brittleness ratio of less than about9.5 (μm)^(−1/2), or a fracture toughness of at least about 0.75MPa·(m)^(1/2).

In a third aspect, the present invention provides a method for making aflexible substrate comprising: a) selecting an amorphous inorganicmaterial capable of forming a substrate having a thickness of less thanabout 250 μm and having at least one of: i) a brittleness ratio of lessthan about 9.5 (μm)^(−1/2), or ii) a fracture toughness of at leastabout 0.75 MPa·(m)^(1/2); and b) forming a substrate from the inorganicmaterial selected in a).

In a fourth aspect, the present invention provides a flexible substratemade by the method described herein.

Additional aspects and advantages of the invention will be set forth, inpart, in the detailed description, figures, and any claims which follow,and in part will be derived from the detailed description or can belearned by practice of the invention. The advantages described belowwill be realized and attained by means of the elements and combinationsparticularly pointed out in the appended claims. It is to be understoodthat both the foregoing general description and the following detaileddescription are exemplary and explanatory only and are not restrictiveof the invention as disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate certain aspects of the presentinvention and together with the description, serve to explain, withoutlimitation, the principles of the invention. Like numbers represent thesame elements throughout the figures.

FIG. 1 illustrates the abraded strength of various materials as afunction of fracture toughness, in accordance with various aspects ofthe present invention.

FIG. 2 illustrates the abraded strength of various materials as afunction of brittleness ratio, in accordance with various aspects of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to thefollowing detailed description, drawings, examples, and claims, andtheir previous and following description. However, before the presentcompositions, articles, devices, and methods are disclosed anddescribed, it is to be understood that this invention is not limited tothe specific compositions, articles, devices, and methods disclosedunless otherwise specified, as such can, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular aspects only and is not intended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its currently known aspects. To this end,those skilled in the relevant art will recognize and appreciate thatmany changes can be made to the various aspects of the inventiondescribed herein, while still obtaining the beneficial results of thepresent invention. It will also be apparent that some of the desiredbenefits of the present invention can be obtained by selecting some ofthe features of the present invention without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations to the present invention are possible andcan even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

Disclosed are materials, compounds, compositions, and components thatcan be used for, can be used in conjunction with, can be used inpreparation for, or are products of the disclosed method andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein. Thus, if a class of substituents A,B, and C are disclosed as well as a class of substituents D, E, and Fand an example of a combination aspect, A-D is disclosed, then each isindividually and collectively contemplated. Thus, in this example, eachof the combinations A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F arespecifically contemplated and should be considered disclosed fromdisclosure of A, B, and C; D, E, and F; and the example combination A-D.Likewise, any subset or combination of these is also specificallycontemplated and disclosed. Thus, for example, the sub-group of A-E,B-F, and C-E are specifically contemplated and should be considereddisclosed from disclosure of A, B, and C; D, E, and F; and the examplecombination A-D. This concept applies to all aspects of this disclosureincluding, but not limited to any components of the compositions andsteps in methods of making and using the disclosed compositions. Thus,if there are a variety of additional steps that can be performed it isunderstood that each of these additional steps can be performed with anyspecific aspect or combination of aspects of the disclosed methods, andthat each such combination is specifically contemplated and should beconsidered disclosed.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “component” includes aspects having two or moresuch components, unless the context clearly indicates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not. For example, the phrase “optional component” means that thecomponent can or can not be present and that the description includesboth aspects of the invention including and excluding the component.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

The following documents describe various compositions and methods fortesting physical properties of compositions, and they are herebyincorporated by reference in their entirety and for the specific purposeof disclosing materials and testing methodologies relating to hardness,fracture toughness, and brittleness ratios: Anstis, G. R. et al. “ACritical Evaluation of Indentation Techniques for Measuring FractureToughness: I, Direct Crack Measurements”, J. Am. Ceram. Soc. 64 (9)533-538 (1981); Lawn, B. R. et al. “Hardness, Toughness, andBrittleness: An Indentation Analysis”, J. Am. Ceram. Soc. 62 (7-8)347-350 (1979); Sehgal, Jeetendra et al. “A New Low-Brittleness Glass inthe Soda-Lime-Silica Glass Family”, J. Am. Ceram. Soc. 81 (9) 2485-2488(1998); Sehgal, Jeetendra et al. “Brittleness of glass”, J.Non-Crystalline Solids 253 (1999) 126-132; and Oliver, W. C. et al. “Animproved technique for determining hardness and elastic modulus usingload and displacement sensing indentation experiments”, J. Mater. Res. 7(6) 1564-1583 (1992).

As briefly introduced above, the present invention provides compositionsfor flexible substrates and specifically mechanically durable, flexiblesubstrates that comprise an amorphous inorganic composition and can beused in electronic devices, such as, for example, light emittingdisplays, including LCD, OLED, electrophoretic, and cholesteric liquidcrystal based display devices, as well as silicon and organicsemiconductor devices, such as photovoltaic, RFID, solar cell, andsensor technology devices. The present invention provides, in part,criteria for selecting substrate materials that can be suitable for usein such electronic devices. The selection criteria and propertiesdescribed herein can be utilized individually or in any combination thatcan provide a suitable substrate.

Electronic applications, such as flexible light emitting displaydevices, can require a substrate to be capable of being bent or oftolerating tensile stresses during, for example, manufacturing and/oruse. Failure of substrates is typically dependent upon the size andconcentration of substrate flaws, the amount of stress placed on thesubstrate, and the substrate material's ability to resist fractures.Various approaches have been attempted to reduce or prevent substratefailure. Such approaches include the addition of coating layers to asubstrate to prevent defects from forming at the substrate surface andminimizing the stress level experienced by a substrate, such as by, forexample, minimizing the modulus of the substrate material, minimizingthe substrate thickness, and/or minimizing the distance between thesubstrate surface and the stress-free neutral axis. The presentinvention provides selection criteria for substrate materials, such asfracture toughness, brittleness ratio, modulus, fatigue strength, andbend radii. Such selection criteria can address a substrate material'sability to resist fractures or strength limiting damage, as well asother intrinsic material properties.

The substrate of the present invention can be any thickness suitable foruse in an electronic device. The substrate can be less than about 250μm, for example, 250, 220, 180, 150, 110, 80, 75, 60, 40, or 30 μm;preferably less than about 150 μm, for example, 140, 120, 100, 80, 75,60, or 40 μm; or more preferably less than about 75 μm, for example, 70,60, 50, 40, or 30 μm. In one aspect, the substrate has a thickness offrom about 1 μm to less than about 250 μm. In another aspect, thesubstrate is about 250 μm. In another aspect, the substrate is about 150μm thick. In yet another aspect, the substrate is about 75 μm thick. Invarious other aspects, the substrate can be about 250 μm or greater.

The primary selection criteria for the substrate material of the presentinvention include fracture toughness and/or brittleness ratio. Asubstrate of the present invention can have a fracture toughness, abrittleness ratio, or both a fracture toughness and a brittleness ratioin accordance with the descriptions and values described herein.

Fracture Toughness

Fracture toughness, as used herein, refers to the ability of a materialcontaining a crack or other defect to resist fracture. Fracturetoughness, denoted as K_(Ic), is typically expressed in units ofMPa·(m)^(1/2). Fracture toughness is a quantitative expression of amaterial's resistance to brittle fracture when a crack is present. Thesubstrate of the present invention can have a fracture toughness of atleast about 0.75 MPa·(m)^(1/2), for example, about 0.75, 0.77, 0.80,0.83, 0.85, 0.87, 0.9, 0.95, 0.99, 1.0, 1.5, or 1.1 MPa·(m)^(1/2);preferably at least about 0.85 MPa·(m)^(1/2), for example, about 0.85,0.87, 0.9, 0.95, 0.99, 1.0, 1.5, or 1.1 MPa·(m)^(1/2); more preferablyat least about 1.0 MPa·(m)^(1/2), for example, about 1.0, 1.05, 1.1,1.15, or 2 MPa·(m)^(1/2); or most preferably at least about 1.1MPa·(m)^(1/2), for example, about 1.1, 1.12, 1.14, 1.16, 1.18, 1.2, or1.3 MPa·(m)^(1/2). In one aspect, the substrate has a fracture toughnessof from at least about 0.75 MPa·(m)^(1/2) to about 10 MPa·(m)^(1/2). Inanother aspect, the substrate of the present invention has a fracturetoughness of about 0.86 MPa·(m)^(1/2). In another aspect, the substrateof the present invention has a fracture toughness of about 0.95MPa·(m)^(1/2).

Brittleness Ratio

Brittleness ratio, as used herein, refers to the ratio of hardness tofracture toughness for a specific material. Brittleness ratio cantypically be expressed as H/K_(Ic) and has the units of (μm)^(−1/2). Amechanically durable, flexible substrate will exhibit a low hardness anda high fracture toughness, thus resulting in a low brittleness ratio. Asubstrate of the present invention can have a brittleness ratio of lessthan about 9.5 (μm)^(−1/2), for example, less than about 9.5, 9.3, 9.1,8.8, 8.5, 8.3, 8.1, 7.9, 7.75, 7.5, 7.25, 7.0, 6.75, 6.5, 6.25, 6, or5.5 (μm)^(−1/2); preferably less than about 8.0 (μm)^(−1/2), forexample, less than about 8.0, 7.9, 7.75, 7.5, 7.25, 7.0, 6.75, 6.5,6.25, 6, or 5.5 (μm)^(−1/2); more preferably less than about 6.5(μm)^(−1/2), for example, less than about 6.5, 6.25, 6, 5.5, 5. or 4.5(μm)^(−1/2); or most preferably less than about 5.5 (μm)^(−1/2), forexample, less than about 5.5, 5.25, 5, 4.75, or 4.5 (μm)^(−1/2). In oneaspect, the substrate has a brittleness ratio of from about 0.1(μm)^(−1/2) to less than about 9.5 (μm)^(−1/2). In another aspect, thesubstrate has a brittleness ratio of about 6.46 (μm)^(−1/2). In anotheraspect, the substrate has a brittleness ratio of about 5.5 (μm)^(−1/2).

The substrate of the present invention can have a fracture toughnessand/or a brittleness ratio as described above. It is not necessary thata substrate have both a fracture toughness of, for example, at leastabout 0.75 MPa·(m)^(1/2), and a brittleness ratio of, for example, lessthan about 9.5 (μm)^(−1/2). In one aspect, a substrate has a fracturetoughness of at least about 0.75 MPa·(m)^(1/2). In another aspect, asubstrate has a brittleness ratio of less than about 9.5 (μm)^(−1/2). Inyet another aspect, a substrate has both a fracture toughness of atleast about 0.75 MPa·(m)^(1/2) and a brittleness ratio of less thanabout 9.5 (μm)^(−1/2).

Modulus×Substrate Thickness

The stress level that a substrate material experiences during bendingcan be proportional to the modulus (E) of the substrate material and tothe distance from the stress free neutral axis. The location of thestress free neutral axis of a particular substrate can vary with thesubstrate composition. The location of the stress free neutral axis canalso vary between single and multilayer substrates, such as those of afabricated device or those comprising a coating material. In anexemplary aspect, a flexible substrate is pulled through a roll-to-rollprocessing system wherein the tensile stress (σ_(t)) is inverselyproportional to the cross sectional area, and thus to the substratethickness. In this exemplary aspect, the total stress on the substrateis the sum of the bending stress experienced while traveling through aroller system and the tensile stress described above.

To achieve the desired durability and flexibility, the product ofmodulus (E) and substrate thickness (t) should be less than about 2GPa·cm, for example, less than about 2.0, 1.8, 1.6, 1.4, 1.2, 1.0, 0.8,0.6, or 0.5 GPa·cm; preferably less than about 1.0 GPa·cm, for example,less than about 1.0, 0.9, 0.7, or 0.5 GPa·cm; or more preferably lessthan about 0.5 GPa·cm, for example, less than about 0.5, 0.4, 0.3, or0.2. A modulus thickness product less than about 2 GPa·cm is notnecessary for the present invention, but can provide improved durabilityand flexibility to a substrate. In one aspect, a substrate has a productof modulus and thickness of from about 0.001 GPa·cm to less than about 2GPa·cm. In another aspect, a substrate has a product of modulus andthickness of about 1.8 GPa·cm. In another aspect, a substrate has aproduct of modulus and thickness of about 1.4 GPa·cm. In yet anotheraspect, a substrate has a product of modulus and thickness of about 0.5GPa·cm. The criteria for modulus thickness can be combined with thefracture toughness value, the brittleness ratio, or both the fracturetoughness value and the brittleness ratio. In one aspect, a substratehas a modulus thickness product of about 1.8 GPa·cm and a fracturetoughness of about 1.0 MPa·(m)^(1/2). In another aspect, a substrate hasa modulus thickness product of about 2.5 GPa·cm and a brittleness ratioof less than about 6.5 (μm)^(−1/2). In yet another aspect, a substratehas a modulus thickness product of about 1.8 GPa·cm, a fracturetoughness of about 0.9 MPa·(m)^(1/2), and a brittles ratio of less thanabout 7.0 (μm)^(−1/2). Glass materials that have traditionally been usedfor flexible substrates, such as, for example, AF45 (Schott), D263(Shott), and 0211 (Corning), can have low modulus thickness products,but typically do not possess the requisite fracture toughness and/orbrittleness ratio.

Fatigue Strength

In flexible substrates, fracture mechanics typically apply to flawspresent in the substrate material. In particular, the stress intensityfactor, K_(I), is related to the surface tensile stress, σ_(a), and flawdepth, a, according to the equationK _(I) =Yσ _(a)(πa)^(1/2)where Y is a geometric factor for a flaw present in the substratematerial. When K_(I) reaches the fracture toughness of the material(K_(I)=K_(IC)), failure occurs. In addition, the relationship betweencrack velocity and stress intensity can be represented by the equationV=AK _(I) ^(n)where both A and n are crack growth parameters. The crack growthparameter, n, can provide an indication of the substrate material'ssusceptibility to subcritical crack growth. For glasses, ceramics, andglass-ceramic materials, n is typically measured using a dynamic fatiguestrength where the material strength, σ_(f), is measured as a functionof stress rate, σ_(r), such as in the equation(σ_(f1)/σ_(f2))^(n+1)=(σ_(r1)/σ_(r2))where subscripts 1 and 2 represent the measured strength for differingrates of stress. The value for n can be determined by simple regressionof log strength versus log stress rate where the slope is equal to1/(n+1). Exemplary fatigue strength values for glass materials, asobtained by the dynamic fatigue method, are detailed in Table 1, below.

TABLE 1 Exemplary Fatigue Strength Values Glass Material n Soda-limesilicate 15 Alkali free, high lead 15 Low alkali display glass 18-29 Eglass 27 Aluminosilicate 27 TiO₂—SiO₂ (8 wt. %) 30 Borosilicate 33Silica 38

Glass materials used in conventional sheet forming processes typicallyhave n values of less than about 30. In contrast, glass materials havingfew network modifiers, such as silica, typically have n values at orexceeding 30. In addition to the fracture toughness and/or brittlenessratio described above, the substrate of the present invention canoptionally have a fatigue value, n, greater than the fatigue value ofglasses typically used in display applications, or of at least about 29,for example, about 29, 30, 31, 33, 35, 38, 39, 40, 42, 46, or 50;preferably of at least about 38, for example, about 38, 39, 40, 42, 46,or 50. A fatigue value of at least about 29 is not necessary, but canprovide improved physical properties and performance to a substrate. Inone aspect, the substrate of the present invention has a fatigue value,n, of 30. In another aspect, the substrate of the present invention hasa fatigue value, n, of 39.

Bend Radius

The bend radius of a flexible substrate is the minimum radius to which asubstrate can be flexed without fracturing. The allowable bend radius ofa flexing substrate is typically inversely proportional to the allowableapplied bend stress. Thus, materials with a higher n value can allow aflexible substrate to bend to a smaller radius. A substrate of thepresent invention can have a bend radius of less than about 30 cm, forexample, less than about 30, 28, 26, 24, 22, 20, 18, 16, 14, 12, 10, 8,6, 4, 2, 1, or 0.5 cm; preferably less than about 10 cm, for example,less than about 10, 8, 6, 4, 2, or 1 cm; or more preferably less thanabout 2 cm, for example, less than about 2, 1.6, 1.2, 1, 0.8, 0.6, 0.4,or 0.3 cm. A bend radius of less than about 30 cm is not necessary for asubstrate of the present invention, but can provide improved flexibilityand performance. In one aspect, a substrate has a bend radius of about26 cm. In another aspect, a substrate has a bend radius of about 8 cm.In yet another aspect, a substrate has a bend radius of about 1.2 cm.

Substrate Composition

The substrate of the present invention comprises at least one amorphousinorganic composition. As used herein, “amorphous” refers tonon-crystalline materials in which there is no long-range order. Theamorphous inorganic composition can be any inorganic compositionsuitable for use in the intended application, such as, for example, anelectronic device, provided that the substrate has at least one of abrittleness ratio or a fracture toughness as described above. Theamorphous inorganic composition can comprise a glass, a glass-ceramic,or a combination thereof. Exemplary glass materials can comprise aboro-silicate glass, a soda-lime glass, a phosphate glass, analumino-borosilicate glass, a zinc borosilicate glass, a silica glass, abarium borosilicate glass, an aluminosilicate glass, an alkaline earthaluminosilicate glass, a rare earth aluminosilicate glass, or acombination thereof. In one aspect, the substrate comprises a glass. Ina specific aspect, the substrate comprises an alumino-borosilicateglass.

The substrate can optionally comprise other compositions. It is notnecessary that the entire substrate be an amorphous inorganiccomposition or that any optional compositions, if present, comprise anamorphous inorganic composition. Such optional compositions, if present,can include crystalline materials. In one aspect, the substratecomprises a glass and a crystalline component. Substrate materials andcompositions, such as, for example, glass materials, for use inpreparing substrates are commercially available and one of skill in theart could readily select an appropriate material and/or compositionbased on the criteria recited herein.

Substrate Coating

The substrate of the present invention can optionally comprise a coatingon at least a portion of at least one substrate surface. A coating canprotect the substrate surface, impart mechanical support to thesubstrate, and/or provide other properties to the substrate. A coating,if present, can comprise any material and be present at any thicknesssuitable for use in an electronic device. A coating can be present as asingle layer or as multiple, for example, 2, 3, 4, 5, or more layers.Multiple layers, if present, can comprise either the same or differentcompositions. It is not necessary that all layers comprise the samecomposition. In one aspect, a substrate comprises a single layer coatingon one substrate surface. In another aspect, a substrate comprises asingle layer coating on two opposing substrate surfaces. In variousexemplary aspects, a coating material comprises polyethylenenaphthalate, polyethersulfone, polycarbonate, polyester, polyethylene,polyarylate, polyolefin, cyclic olefin copolymer, polyarylamine,polyamide, polyimide, or a combination thereof. In one aspect, thesubstrate comprises a polyarylate coating. In another aspect, thesubstrate comprises a polyethersulfone coating. A coating, if present,can be any thickness suitable for the intended application. The optionalcoating can be from less than about 1 μm to about 200 μm, or more, forexample, about 0.3, 1, 2, 4, 7, 10, 14, 20, 30, 50, 70, 100, 120, 140,160, 190, 200, or 220 μm. In one aspect, the substrate comprises acoating having a thickness of 1 μm. In another aspect, the substratecomprises a coating having a thickness of 30 μm. In yet another aspect,the substrate comprises a coating having a thickness of 150 μm. In afurther aspect, a substrate comprises a two layer coating on twoopposing surfaces of the substrate. Coatings and coating materials arecommercially available and one of skill in the art could readily selectand apply an appropriate coating to a substrate based on the method ofdevice fabrication and/or the intended application.

Electronic Device

The substrate of the present invention can be utilized in a variety ofelectronic devices, such as, for example, a light emitting displaydevice. The design of a device can vary depending on the intendedapplication and requirements. A device can be flexible or cannecessitate that at least a portion of a substrate be flexible. In oneaspect, the electronic device is a light emitting device, such as, forexample, an organic light emitting display device. In another aspect,the device has a flexible substrate capable of a bend radius of lessthan about 30 cm.

Devices requiring mechanically durable and flexible substrates caninclude applications having size and/or weight limitations, such as, forexample, cell phones and laptop computers. In such applications, thesubstrate may remain flat during both the device manufacturing processand in the final application. Such designs can require that a substratebe capable of withstanding bending stresses, but this requirement is notpresent in all applications. While a substrate of the present inventioncan be capable of a bend radius of less than about 30 cm, the mechanicaldurability properties provide additional benefit and value because thesubstrate can be thinner and lighter weight than conventional substratematerials.

Other applications of mechanically durable substrates include deviceswhere the substrate can undergo a bend radius either during devicefabrication or during final application. Examples of such devicesinclude both electronic and display applications such as solar cells,photovoltaics, organic light emitting diode displays, electrophoreticdisplays, LCD displays, cholesteric liquid crystal displays, Si TFTelectronics, organic TFT electronics, oxide based electronics, and otherdevice technologies. In such devices, a substrate can undergo a bendradius during device fabrication, such as, for example, in abonding/de-bonding step to a carrier substrate, use of a roll-to-rollfabrication process, use of a sheet fed continuous fabrication process,installation of large area displays or electronics requiring un-rollingof a rolled substrate, use of a spooling or un-spooling process, orother processes involving a bend radius. Exemplary devices requiring abend radius include scrollable, foldable, hinged, or other devices thatcan experience variable bend states during the device lifetime.Additional exemplary devices include conformable display or electronicdevices, such as, for example, display or electronics for automotivedashboards, airplane cockpits, lighting, architectural devices, sensors,and other devices that can be bent once to a permanent or semi-permanentstate during the device lifetime.

Method of Making a Flexible Substrate

The present invention further comprises a method for making a flexiblesubstrate comprising: selecting an inorganic material capable of forminga substrate having a thickness of less than about 250 μm and having atleast one of: a) a brittleness ratio of less than about 9.5 (μm)^(−1/2),or b) a fracture toughness of at least about 0.75 MPa·(m)^(1/2); andthen forming a substrate from at least the selected amorphous inorganicmaterial. The forming process can comprise a sintering process, aconsolidation process, a drawing process, a process involving aninorganic melt, a slot draw process, a fusion draw process, an updrawprocess, an overflow process, a downdraw process, a re-draw process, ablowing process, a float process, a crystallization process, anannealing process, a soot deposition process, a roll forming process,other processes capable of forming a thickness of less than about 250μm, other processes capable of affecting the intrinsic properties (e.g.,fracture toughness, modulus, brittleness ratio, fatigue resistance) ofan already formed inorganic sheet or article, or a combination thereof.In one aspect, the forming process comprises a fusion process. Inanother aspect, the forming process comprises a sintering process. Inyet another aspect, the forming process comprises a downdraw process.Various forming processes are known and one of skill in the art couldreadily select an appropriate forming process for use in manufacturing asubstrate composition in accordance with the present invention.

The various approaches described herein can be used individually, or inany combination, to form a flexible substrate or an electronic devicecomprising a flexible substrate. In various aspects, a substrate has abrittleness ratio less than about 9.5 (μm)^(−1/2), a fracture toughnessof at least about 0.75 MPa·(m)^(1/2), a product of modulus and thicknessof less than about 2 GPa·cm, the capability of achieving a bend radiusof less than about 30 cm without fracture, or a combination thereof.

Although several aspects of the present invention have been illustratedin the accompanying figures and described in the detailed description,it should be understood that the invention is not limited to the aspectsdisclosed, but is capable of numerous rearrangements, modifications andsubstitutions without departing from the spirit of the invention as setforth and defined by the following claims.

EXAMPLES

To further illustrate the principles of the present invention, thefollowing examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thearticles, devices, and methods claimed herein are made and evaluated.They are intended to be purely exemplary of the invention and are notintended to limit the scope of what the inventors regard as theirinvention. Efforts have been made to ensure accuracy with respect tonumbers (e.g., amounts, temperatures, etc.); however, some errors anddeviations should be accounted for. Material properties can exhibitvariability depending upon, for example, the particular batch andvendor. Data distributions are thus expected in any measurement method.Unless indicated otherwise, temperature is ° C. or is at ambienttemperature, and pressure is at or near atmospheric. There are numerousvariations and combinations of process conditions that can be used tooptimize product quality and performance. Only reasonable and routineexperimentation will be required to optimize such process conditions.

Example 1—Determination of Substrate Properties

In a first example, modulus, hardness, fracture toughness, andbrittleness ratios were determined for a variety of materials, includingceramics, glass ceramics, silica glasses, phosphate glasses, and othercompositions, as detailed in Table 2, below. Various materials areincluded to illustrate the physical properties that are suitable for usein substrates in accordance with the present invention, although not allof the materials identified in Table 2 can readily form a glass. Modulusvalues refer to nano-indentation measurements. Hardness values refer tonano-indentation, Knoop, and/or Vickers measurements, wherein anindentation is made in the substrate by, for example, a pyramidal shapeddiamond. Fracture toughness values refer to indentation and/or Chevronnotch measurements. Brittleness ratio values refer to measurements ofhardness and fracture toughness, as described above. Nano-indentation,Knoop hardness, Vickers hardness, and Chevron notch measurements areknown and one of skill in the art could readily select an appropriatetest methodology to determine the modulus, hardness, fracture toughness,and/or brittleness ratio of a particular material.

The values in Table 2, include both literature values, where available,and experimentally obtained values. In some cases the general materialproperty values were taken from open literature where the specificmeasurement technique was not specified. In these cases the valuesshould be assumed to be approximate.

TABLE 2 Properties of Materials Fracture Brittleness Abraded HardnessToughness Ratio Strength Material Modulus GPa GPa MPa · (m)^(1/2)(μm)^(−1/2) MPa Tungsten Carbide 680.0 16.97 8.9 1.91 Aluminum Oxide300.0 24.09 6.54 3.68 222 Sapphire 468.0 22.54 3.45 6.53 403 AluminumNitride 323.0 11.61 3.27 3.55 111 Silicon Nitride 312.0 14.60 3.12 4.68Titanium Dioxide 270.0 11.00 2.80 3.93 Corning Macor ® 66.9 2.45 1.531.60 102 Corning 7740 61.7 6.20 0.96 6.46 55 Schott BK-7 82.0 5.66 0.866.58 Corning 1737 70.4 5.59 0.78 7.16 55 Corning HPFS ® 72.7 4.88 0.756.51 43 Corning 0211 74.4 7.70 0.71 10.85 Schott D263T 72.3 7.40 0.6711.04 Corning 7070 49.2 6.50 0.64 10.09 38 Corning 1779 87.7 4.59 0.647.17 44 Schott AF45 65.2 6.39 0.64 9.98 Corning 0215 70.0 5.68 0.63 9.0242 Saint-Gobain CS77 81.9 6.61 0.56 11.76 43 Alkali fluorophosphate 70.06.20 0.43 14.42 22 glass* *such as an alkali zinc fluorophosphate of thepyrophosphate family

As illustrated in Table 2, a number of conventional glass materials donot possess the fracture toughness or brittleness ratio required by theselection criteria of the present invention. Further, as indicatedabove, some of the materials detailed in Table 2 that possess either therequired fracture toughness and/or brittleness ratio do not readily formglasses and thus, may not be suitable for many applications. Some of theillustrated materials, such as, for example, Corning Macor®, Corning7740, Schott BK-7, Corning 1737, and Corning HPFS® possess both afracture toughness of at least about 0.75 MPa·(m)^(1/2) and/or abrittleness ratio of less than about 9.5 (μm)^(−1/2), in accordance withthe selection criteria of the present invention. These materials arethus suitable for use as compositions in the thin flexible substrates ofthe present invention.

Example 2—Abraded Strength

In a second example, abraded strength values were determined for some ofthe materials described in Example 1. The experimental materials wereselected from drastically differing composition families in order toillustrate the relationship between physical properties. Samples ofselected materials were cut and polished to approximately 1 inch by 1inch pieces, approximately 1 mm thick. The samples were subsequentlyabraded on one side with 150J sandpaper, after which ring-on-ringbiaxial strength was tested with the abraded sample side being put intension. For this strength test, the load ring diameter was typically0.25 inch and the support ring diameter was typically 0.50 inch. A testspeed was typically 0.05 inch per minute.

FIGS. 1-2 illustrate the resulting abraded strength values as a functionof fracture toughness and brittleness ratio, respectively. FIG. 1illustrates a trend of increasing abraded strength with increasingfracture toughness. Similarly, FIG. 2 illustrates the trend ofincreasing abraded strength with decreasing brittleness ratio. Thus, asubstrate having an increased abraded strength can be achieved by eitheran increase in fracture toughness, a decrease in brittleness ratio, or acombination thereof.

Various modifications and variations can be made to the compositions,articles, devices, and methods described herein. Other aspects of thecompositions, articles, devices, and methods described herein will beapparent from consideration of the specification and practice of thecompositions, articles, devices, and methods disclosed herein. It isintended that the specification and examples be considered as exemplary.

What is claimed is:
 1. A substrate comprising an amorphous inorganiccomposition free of Li, Na, K, Rb, Cs, and Fr, wherein the substrate hasa thickness of from about 1 μm to less than about 250 μm and at leastone of: a) a brittleness ratio of less than about 9.5 (μm)^(−1/2), or b)a fracture toughness of at least about 0.75 MPa·(m)^(1/2); wherein thesubstrate has a bend radius of less than about 30 cm; and wherein thesubstrate further comprises at least one coating layer on at least aportion of at least one surface of the substrate, wherein the coatinglayer is selected from the group consisting of a polyethylenenaphthalate, a polyethersulfone, a polycarbonate, a polyester, apolyethylene, a polyarylate, a polyolefin, a cyclic olefin copolymer, apolyarylamine, a polyamide, a polyimide and combinations thereof.
 2. Thesubstrate of claim 1, wherein the coating has a thickness of from lessthan about 1 μm to about 200 μm.
 3. The substrate of claim 2, whereinthe coating has a thickness of about 1 μm.
 4. The substrate of claim 1,wherein the substrate comprises a two layer coating on two opposingsurfaces of the substrate.
 5. The substrate of claim 1, wherein thesubstrate has a modulus, and wherein the product of the substratemodulus and the substrate thickness is less than about 2.0 GPa·cm. 6.The substrate of claim 5, wherein the product of the substrate modulusand the substrate thickness is less than about 1.0 GPa·cm.
 7. Thesubstrate of claim 1, wherein the substrate has both a brittleness ratioof less than about 9.5 (μm)^(−1/2) and a fracture toughness of at leastabout 0.75 MPa·(m)^(1/2).
 8. The substrate of claim 1, wherein thesubstrate has both a brittleness ratio of less than about 9.5(μm)^(−1/2) and a fracture toughness of at least about 0.75MPa·(m)^(1/2); wherein the substrate has a modulus, and wherein theproduct of the substrate modulus and the substrate thickness is lessthan about 2.0 GPa·cm.
 9. The substrate of claim 1, wherein thesubstrate has both a brittleness ratio of less than about 8.0 (μm)^(1/2)and a fracture toughness of at least about 0.9 MPa·(m)^(1/2); whereinthe substrate has a modulus, and wherein the product of the substratemodulus and the substrate thickness is less than about 2.0 GPa·cm. 10.The substrate of claim 1, wherein the substrate has a fatigue value, n,greater than about
 29. 11. The substrate of claim 1, wherein thesubstrate has a fatigue value, n, greater than about
 38. 12. Thesubstrate of claim 1, wherein the composition comprises a glass, aglass-ceramic, or a combination thereof.
 13. The substrate of claim 1,wherein the substrate has a bend radius of less than about 2 cm.
 14. Thesubstrate of claim 1, wherein the substrate is fusion-drawn.
 15. Thesubstrate of claim 1, wherein the substrate is a borosilicate glass. 16.The substrate of claim 15, wherein the borosilicate glass is a analumina-borosilicate glass.
 17. An electronic device comprising anamorphous inorganic composition free of Li Na, K, Rb, Cs, and Fr,wherein the substrate has a thickness of from about 1 μm to less thanabout 250 μm and at least one of: a) a brittleness ratio of less thanabout 9.5 (μm)^(−1/2), or b) a fracture toughness of at least about 0.75MPa·(m)^(1/2); wherein the substrate has a bend radius of less thanabout 30 cm; and wherein the substrate further comprises at least onecoating layer on at least a portion of at least one surface of thesubstrate, wherein the coating layer is selected from the groupconsisting of a polyethylene naphthalate, a polyethersulfone, apolycarbonate, a polyester, a polyethylene, a polyarylate, a polyolefin,a cyclic olefin copolymer, a polyarylamine, a polyamide, a polyimide andcombinations thereof.
 18. The electronic device of claim 17, wherein thesubstrate has a modulus, and wherein the product of the substratemodulus and the substrate thickness is less than about 2.0 GPa·cm. 19.The electronic device of claim 17, wherein the electronic devicecomprises a light emitting display.
 20. The electronic device of claim17, wherein the electronic device comprises an organic light emittingdisplay device.